Radio Frequency (RF) transmitters on board of spacecraft do not always run at maximum power level. When operated at reduced RF power, the consumption of a RF High Power Amplifier (HPA) can be significantly reduced by lowering the voltage of its power supply.
The current state-of-the-art power supply for HPAs is referred to as “flexible” and consists in powering amplifiers with discrete preselected DC voltages. See e.g.:                Darbandi et al., “Flexible S-band SSPA for Space Application”, NASA/ESA Conference on Adaptive Hardware and Systems, 2008; and        C R Green et al., “High Efficiency 240 W GaN SSPA Demonstrator for Space Applications” ESA-MoD GaN round Table, September 2010, ESTEC, The Netherlands.        
The power supply level is not optimized at any time of the HPA operation and the power saving is therefore limited.
A more advanced approach, which is the subject of an intense research effort, is referred to as “envelope tracking”. It consists in providing the HPA with a continuously variable DC voltage, following (“tracking”) the envelope of the RF signal to be amplified. This solution works well when the envelope bandwidth is limited to a few hundreds of kHz up to some MHz—e.g. in the case of terrestrial mobile systems (e.g. base stations, mobile phones). However, space applications have large bandwidths (20-100 MHz); therefore efficient envelope tracking requires a very fast DC/DC converter.
For achieving high efficiency, the DC/DC converter should be a switching converter, or chopper. Moreover, telecommunication satellites generally use a 100 V power bus and a HPA power supply at 25-50 V, which imposes the use of a step-down converter. In practice, the most suitable topology is the “buck” one, and a RF field effect transistor (FET) has to be used as the switching element of the converter, due to its low parasitic capacitances. Moreover, the transistor has to be of N-type, because high-frequency, high-voltage P-type transistors are not commercially available, and in any case would have poor performances.
Using a N-type FET as the switching element of a buck converter, however, implies having its source alternatively connected to the input voltage and to the ground, and therefore experiencing fast variations of its voltage level. This is not a serious problem as long as the input voltage does not exceed a few volts, which allows directly driving the floating gate of the transistor. So, very high-frequency DC/DC converters using both GaAs Depletion and Enhanced mode transistors have been disclosed, see e.g.:                V. Pala et al., “Integrated High-Frequency Power Converters Based on GaAs pHEMT: Technology Characterization and Design Examples”, IEEE Transactions on Power Electronics, May 2012;        V. Pala et al., “Application of GaAs pHEMT Technology for Efficient High Frequency Switching Regulators”, Proceedings of The 22nd International Symposium on Power Semiconductor Devices & ICs, Hiroshima, 2010; and        E. Busking et al., “1 GHz GaAs Buck Converter for High Power Amplifier Modulation Applications”, European Microwave Integrated Circuits Conference, Amsterdam, October 2012.        
At higher voltage levels, however, the floating gate has to be driven through an isolation transformer. But then, at very high switching frequency, the parasitic capacitance between transformer coils will lead peak current during switching whose amplitudes become incompatible with proper converter operation.
This problem would not arise with P-type transistors. However, RF transistors capable of handling voltages of several tens or volts or more (Silicon, GaAs or GaN technology) are only available in N-type.
A possible solution to avoid this difficulty consists in using a switching converter driven at a significantly lower frequency for tracking a low-pass filtered version of the envelope. See e.g. EP 2 432 118. Clearly, this results in a reduced efficiency.
Other alternative solutions have been proposed                Implementing multisource power supply with fast discrete commutation between each DC voltage to follow the envelope variations. See e.g. FR2799063 and S. Forestier et al., “Development of a new dynamic bias control system to increase the power added efficiency and the linearity of a power amplifier for M-QAM modulation”, ESA Microwave Workshop on Microwave techniques and technologies, 2005, ESTEC, The Netherlands.        Using a power converter operated at reduced switching frequency, ensures the envelope tracking up to a given upper frequency, in combination with high bandwidth linear regulator. See e.g. J. Jeong et al., “High-Efficiency WCDMA Envelope Tracking Base-Station Amplifier Implemented With GaAs HVHBTs”, IEEE Journal of Solid-State Circuits, Vol. 44, N° 10, October 2009 or M. Hassan et al., “A Wideband CMOS/GaAs HBT Envelope Tracking Power Amplifier for 4G LTE Mobile Terminal Applications”, IEEE Transactions on Microwave Theory and Techniques, Vol. 60, N° 5, May 2012.        Using larger number of converters (e.g. 10) operated at proportionally lower switching frequency. See e.g. J. Strydom, “eGaN FET-Silicon Power Shoot-Out Volume 8: Envelope Tracking”, Power Electronics Technology, May 2012 or Mark Norris et al, “10 MHz Large Signal Bandwidth, 95% Efficient Power Supply for 3G-4G Cell Phone Base Stations”, Twenty-Seventh Annual IEEE on Applied Power Electronics Conference and Exposition (APEC), 2012.        
However they all lead to complex and expensive systems, with a much lower efficiency than that which could be achieved in principle using a simpler buck converter.
Radio-frequency amplifiers with broadband envelope tracking have been realized using a fast step-up DC/DC converter with “boost” topology. See:                F. Leroy et al., “Experimental Demonstration of High Frequency Switching Converter for Envelope Tracking Power Amplifier Applications”, 63rd International Astronautical Congress, Naples, Italy, October 2012        N. Le Gallou et al., “Over 10 MHz Bandwidth Envelope-Tracking DC/DC converter for Flexible High Power GaN Amplifiers”, IEEE International Microwave Symposium 2011        
Indeed, when a N-type transistor is used as the switching element of a boost converter, its source is connected to the ground, and therefore no isolation transistor is required to drive its gate. However, this approach is only possible in applications having a bus voltage which is lower than the voltage of the power supply of the HPA. This can be the case in scientific spacecraft, but not in telecommunication satellites. Moreover, boost converters are notoriously difficult to control because they are non-minimum phase.
The invention aims at solving the aforementioned drawbacks of the prior art.